Fluctuations in ionic currents flowing across cell membranes during excitation are measured. From analysis of the spectrum of the fluctuations we can estimate the conductance and relaxation times associated with the elementary ionic channels which are activated to cause excitation. The primary experimental preparation is chick skeletal muscle grown in tissue culture. A two-microelectrode voltage clamp is used to measure electrical noise caused by the interaction of acetycholine with the postsynaptic membrane. Power spectra are obtained by on-line computation, and the experimental spectra are compared with predictions derived from theoretical molecular models of the activation, or gating process. The excitation noise which has been measured is a unique physical property of excitable cell membranes and provides a means for studying molecular phenomena underlying excitation. The present system is a prototype for the extension of the technique to other excitable cells such as nerve axons and sensory receptors.